PGD (Preimplantation Genetic Diagnosis) vs PGS (Preimplantation Genetic Screening)

Margaret Graf Garrisi, M.D.
Dr. Garrisi ART

IRMS defines this state-of-the-art lab technology to isolate normal embryos

Genetic testing of embryos for chromosomal or genetic abnormalities has been possible in IVF laboratories since the early 1990’s. At that time, the term “PGD” was used for all types of this testing, not differentiating between chromosomal abnormalities or a disease. Before long, the terminology evolved that the term PGD was reserved for genetic testing of an embryo for a specific genetic disease such as cystic fibrosis or sickle cell disease.

All normal human embryos should have 46 chromosomes; 2 chromosomes numbered 1 through 22 = 44 chromosomes, plus XX (46,XX; female) or XY (46,XY; male). Abnormal chromosomes occur in embryos most often due to maternal aging and can result in the birth of a child with disabilities (i.e. Trisomy 21-yielding a child with Down Syndrome) or a pregnancy loss/ miscarriage (ie. Trisomy 15). Normal embryos have the proper number of chromosomes (46) and this in genetics terms is referred to as “euploid”. Abnormal embryos have extra or missing chromosomes and this is called “aneuploidy”. PGS is the testing to see if the number of chromosomes an embryo has is normal.

In the early days of this technology embryos were biopsied on day 3 after egg retrieval and usually 1 cell was available to be removed from a 6-8 celled embryo. If the couple were doing PGD for a specific disease, they would normally choose to do PGD and hope that if the embryo was not affected with the disease that its chromosomes would also be normal. Now, almost all embryo biopsies for PGD or PGS are done on blastocyst embryos (on day 5, 6 or even day 7 after retrieval). These embryos usually have > 100 cells and about 5 to 10 cells are biopsied. This is then much less stressful for the embryo. For routine maternal aging or recurrent pregnancy loss, PGS is the type of genetic testing that would be done. But now that more cells are available for testing , a couple who are carriers for diseases such as cystic fibrosis or sickle cell disease can have simultaneous testing for both PGS and PGD on the same embryo to allow the transfer of an embryo that does not have the disease and also has a normal chromosome complement.

There are different types of laboratory techniques for these tests such as PGS. State-of-the-art lab technology includes CGH (Comparative Genomic Hybridization) and NGS (Next Generation Sequencing), which in PGS allow testing of all the chromosomes and chromosomal abnormalities. At IRMS, our lab performs both CGH and NGS on a daily basis.

If you wish to learn more about the genetic testing that can be done with IVF or seek to speak with a specialist at IRMS please call us at (973) 322–8286 or fill out our contact form. We look forward to helping you.

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